1. Field of the Invention
The present invention relates to a prosthesis, more specifically an artificial bone connection prosthesis used for the replacement of damaged bones, an articular cartilage such as a coxa and a knee joint, and a spine's intervertebral disc.
In the disclosure described below, the invention relating to the articular cartilage is described as a first invention and the invention relating to the intervertebral discs is described as a second invention.
2. Prior Art
At present, even if an articular cartilage is partially damaged, it is replaced with an artificial articular cartilage since there is no appropriate surgical treatment method. The artificial articular cartilage is greatly beneficial to patients who are suffering from joint trouble. However, serious problems, such as complications caused by the replacement of artificial joints, in particular a high occurrence rate of loosening problems and the necessity of another operation due to the breakage of the bones around the artificial joints.
The artificial joints which are currently used are made of hard materials such as metals and ceramics and a soft socket made of ultra-polymer high-density polyethylene (HDPE). This HDPE is superior in wear resistance, but it is inferior in wear resistance and shock absorbing performance for impact load. Reaction of foreign matter caused by powder generated due to wear is also causing problems.
An articular cartilage is located on the surface of a bone joint. The articular cartilage of the knee joint or coxa of a human being is 2 to 4 mm in thickness. The articular cartilage absorbs shock applied by the weight of the human being and distributes and transmits the impact load to a chondral cancellous bone trabeculation to play an important role of allowing smooth movability under low friction. One of the most important features of the articular cartilage is its high extracellular water content, 60 to 80%. The elasticity and lubrication of the articular cartilage are brought by this high water content.
So far, various materials, such as HDPE, polyhydroxyethyl methacrylate (PHEMA) gel, silicone and polyvinyl alcohol (PVA) gel, have been examined as the materials for the artificial articular cartilage, but they were insufficient in their dynamic characteristics.
The materials of the artificial articular cartilage must meet the requirements listed below:
i) Superior affinity to a human body PA0 ii) Chemical and physical stability PA0 iii) Superior dynamic characteristics (wear resistance, shock absorbing effect, lubrication, etc.)
As described above, conventional artificial articular cartilages have-drawbacks in characteristics regarding lubrication, shock absorbing and wear resistance.
On the other hand, an intervertebral disc is located between individual vertebral bodies. More specifically, two vertebral bodies and an intervertebral disc located therebetween comprise a motion segment: a function unit at the spine. The disc comprises sturdy fiber rings which surround the outer circumference of the nucleus pulposus made of fiber cartilage with alternately arranged collagen fiber bundles, liquefied soft nucleus pulposus which holds the tension state of the intervertebral disc made of colloidal gel including much glycosaminoglycan bonded to water, and the end plate of cartilage which is made of glassy cartilage and is used to separate the vertebral body from the above-mentioned elements.
In our daily life, the intervertebral disc is subjected to complicated load which is a combination of compression, bending and distortion. When the spine is bent forward, stretched and bent sideways, stresses, in particular, tension and compression stresses occur on the disc. When the spine is turned, shearing stress occurs on the disc. The load applied to the nucleus pulposus in the intervertebral disc is uniformly distributed in a normal case and exerts force hydrostatically. Therefore, the intervertebral disc has a hydrostatic function at its motion segment. The disc thus has very important roles; it functions as a cushion between vertebral bodies, stores energy and absorbs shock of load by distributing the load. In addition, the disc is greatly related to the internal stability together with ligaments.
If the intervertebral disc having the above-mentioned important functions is damaged by a disease such as the hernia of the intervertebral disc, the damaged disc is treated as described below at present. The deformed intervertebral disc is taken out first. Then, a self transplantation bone, bone cement or an artificial vertebral body spacer is mainly used to fix the portion between the vertebral bodies.
However, all means, including the self transplantation bone, artificial vertebral body made of stainless steel, alumina ceramics and hydroxy apatite, bone cement made of polymethyl methacrylate or a combination of these, are used only to fix the connection between vertebral bodies without considering the dynamic characteristics inherent to the intervertebral disc of a living body. The flexibility and shock absorbing function inherent to the intervertebral disc are disregarded. In particular, the upper and lower sections adjacent to the intervertebral disc are adversely affected in the living body dynamics, thereby frequently causing secondary problems at the sections adjacent to the intervertebral disc.
In addition to the above-mentioned technologies practiced actually, the prior arts disclosed so far are: "Artificial vertebral body spacer" (Japanese Provisional Patent Publication No. 64-76851) comprising a metal mesh unit with an elasticity close to that of the bone and being characterized in the method of fixing the unit to the upper and lower vertebral bodies, "Artificial intervertebral disc for a cervical vertebra" (Japanese Provisional Patent Publication No. 63-164948) attaching importance to the motion of the intervertebral disc, "Artificial vertebral body" (Japanese Provisional Patent Publication No. 63-300758) aiming for substitution of the dynamic characteristics of the intervertebral disc, "Artificial intervertebral disc capsule and its embedding method" (Japanese Provisional Patent Publication No. 64-70041) and "Intervertebral disc prosthesis" (Japanese Provisional Patent Publication No. 2-215461).
The various artificial intervertebral discs devised so far are only artificial vertebral body spacers used just to substitute self transplantation bones and had no dynamic characteristics of the intervertebral disc of the living body, or have insufficient internal stability as a result that too much importance is placed to mobility. Even composite body discs aimed to substitute the dynamic characteristics of the intervertebral disc, made of flexible polymer material mainly at the inner sections and comprising hard materials, such as ceramic powder material or bioinactive fibers, at the sections contacting bones cannot sufficiently substitute the softness and shock absorbing function of the intervertebral disc-of the living body, cannot be sufficiently fixed to the bones or cannot have sufficient mechanical strength of itself. In this way, these composite body discs cannot satisfy the above-mentioned overall functions at the same time as artificial intervertebral discs.